The era of tumor immunology began with experiments by Prehn and Main, who showed that antigens on the methylcholanthrene (MCA)-induced sarcomas were tumor specific in that transplantation assays could not detect these antigens in normal tissue of the mice (Prehn, R. T., et al., 1957, J. Natl. Cancer Inst. 18:769-778). This notion was confirmed by further experiments demonstrating that tumor specific resistance against MCA-induced tumors can be elicited in the autochthonous host, that is, the mouse in which the tumor originated (Klein, G., et al., 1960, Cancer Res. 20:1561-1572).
In subsequent studies, tumor specific antigens were also found on tumors induced with other chemical or physical carcinogens or on spontaneous tumors (Kripke, M. L., 1974, J. Natl. Cancer Inst. 53:1333-1336; Vaage, J., 1968, Cancer Res. 28:2477-2483; Carswell, E. A., et al., 1970, J. Natl. Cancer Inst. 44:1281-1288). Since these studies used protective immunity against the growth of transplanted tumors as the criterion for tumor specific antigens, these antigens are also commonly referred to as “tumor specific transplantation antigens” or “tumor specific rejection antigens.” Several factors can greatly influence the immunogenicity of the tumor induced, including, for example, the specific type of carcinogen involved, immunocompetence of the host and latency period (Old, L. J., et al., 1962, Ann. N.Y. Acad. Sci. 101:80-106; Bartlett, G. L., 1972, J. Natl. Cancer Inst. 49:493-504).
Most, if not all, carcinogens are mutagens which may cause mutation, leading to the expression of tumor specific antigens (Ames, B. N., 1979, Science 204:587-593; Weisburger, J. H., et al., 1981, Science 214:401-407). Some carcinogens are immunosuppressive (Malmgren, R. A., et al., 1952, Proc. Soc. Exp. Biol. Med. 79:484-488). Experimental evidence suggests that there is a constant inverse correlation between immunogenicity of a tumor and latency period (time between exposure to carcinogen and tumor appearance) (Old, L. J., et al., 1962, Ann. N.Y. Acad. Sci. 101:80-106; and Bartlett, G. L., 1972, J. Natl. Cancer Inst. 49:493-504). Other studies have revealed the existence of tumor specific antigens that do not lead to rejection, but, nevertheless, can potentially stimulate specific immune responses (Roitt, I., Brostoff, J and Male, D., 1993, Immunology, 3rd ed., Mosby, St. Louis, pps. 17.1-17.12).
2.1. Tumor-Specific Immunogenicities of Heat Shock/Stress Proteins hsp70, hsp90 and gp96
Srivastava et al. demonstrated immune response to methylcholanthrene-induced sarcomas of inbred mice (1988, Immunol. Today 9:78-83). In these studies it was found that the molecules responsible for the individually distinct immunogenicity of these tumors were identified as cell-surface glycoproteins of 96 kDa (gp96) and intracellular proteins of 84 to 86 kDa (Srivastava, P. K., et al., 1986, Proc. Natl. Acad. Sci. USA 83:3407-3411; Ullrich, S. J., et al., 1986, Proc. Natl. Acad. Sci. USA 83:3121-3125. Immunization of mice with gp96 or p84/86 isolated from a particular tumor rendered the mice immune to that particular tumor, but not to antigenically distinct tumors. Isolation and characterization of genes encoding gp96 and p84/86 revealed significant homology between them, and showed that gp96 and p84/86 were, respectively, the endoplasmic reticular and cytosolic counterparts of the same heat shock proteins (Srivastava, P. K., et al., 1988, Immunogenetics 28:205-207; Srivastava, P. K., et al., 1991, Curr. Top. Microbiol. Immunol. 167:109-123). Further, hsp70 was shown to elicit immunity to the tumor from which it was isolated but not to antigenically distinct tumors. However, hsp70 depleted of peptides was found to lose its immunogenic activity (Udono, M., and Srivastava, P. K., 1993, J. Exp. Med. 178:1391-1396). These observations suggested that the heat shock proteins are not immunogenic per se, but are carriers of antigenic peptides that elicit specific immunity to cancers (Srivastava, P. K., 1993, Adv. Cancer Res. 62:153-177).
2.2. Pathobiology of Cancer
Cancer is characterized primarily by an increase in the number of abnormal cells derived from a given normal tissue, invasion of adjacent tissues by these abnormal cells, and lymphatic or blood-borne spread of malignant cells to regional lymph nodes and to distant sites (metastasis). Clinical data and molecular biologic studies indicate that cancer is a multistep process that begins with minor preneoplastic changes, which may under certain conditions progress to neoplasia.
Pre-malignant abnormal cell growth is exemplified by hyperplasia, metaplasia, or most particularly, dysplasia (for review of such abnormal growth conditions, see Robbins and Angell, 1976, Basic Pathology, 2d Ed., W.B. Saunders Co., Philadelphia, pp. 68-79.) Hyperplasia is a form of controlled cell proliferation involving an increase in cell number in a tissue or organ, without significant alteration in structure or function. As but one example, endometrial hyperplasia often precedes endometrial cancer. Metaplasia is a form of controlled cell growth in which one type of adult or fully differentiated cell substitutes for another type of adult cell. Metaplasia can occur in epithelial or connective tissue cells. Atypical metaplasia involves a somewhat disorderly metaplastic epithelium. Dysplasia is frequently a forerunner of cancer, and is found mainly in the epithelia; it is the most disorderly form of non-neoplastic cell growth, involving a loss in individual cell uniformity and in the architectural orientation of cells. Dysplastic cells often have abnormally large, deeply stained nuclei, and exhibit pleomorphism. Dysplasia characteristically occurs where there exists chronic irritation or inflammation, and is often found in the cervix, respiratory passages, oral cavity, and gall bladder.
The neoplastic lesion may evolve clonally and develop an increasing capacity for invasion, growth, metastasis, and heterogeneity, especially under conditions in which the neoplastic cells escape the host's immune surveillance (Roitt, I., Brostoff, J and Male, D., 1993, Immunology, 3rd ed., Mosby, St. Louis, pps. 17.1-17.12).
2.3. Immunotherapy
Four basic cell types whose function has been associated with antitumor cell immunity and the elimination of tumor cells from the body are: i) B-lymphocytes which secrete immunoglobulins into the blood plasma for identifying and labeling the nonself invader cells; ii) monocytes which secrete the complement proteins which are responsible for lysing and processing the immunoglobulin-coated target invader cells; iii) natural killer lymphocytes having two mechanisms for the destruction of tumor cells-antibody-dependent cellular cytotoxicity and natural killing; and iv) T-lymphocytes possessing antigen-specific receptors and each T-lymphocyte clone having the capacity to recognize a tumor cell carrying complementary marker molecules (Schreiber, H., 1989, in Fundamental Immunology (ed). W. E. Paul, pp. 923-955).
Several factors can influence the immunogenicity of tumors induced. These factors include dose of carcinogen, immunocompetence of the host, and latency period. Immunocompetence of the host during the period of cancer induction and development can allow the host to respond to immunogenic tumor cells. This may prevent the outgrowth of these cells or select far less immunogenic escape variants that have lost their respective rejection antigen. Conversely, immunosuppression or immune deficiency of the host during carcinogenesis or tumorigenesis may allow growth of highly immunogenic tumors (Schreiber, H., 1989, in Fundamental Immunology (ed). W. E. Paul, pp. 923-955).
Three major types of cancer immunotherapy are currently being explored: i) adoptive cellular immunotherapy, ii) in vivo manipulation of patient plasma to remove blocking factors or add tumoricidal factors, and iii) in vivo administration of biological response modifiers (e.g., interferons (IFN; IFN-alpha and IFN-gamma), interleukins (IL; IL-2, IL-4 and IL-6), colony-stimulating factors, tumor necrosis factor (TNF), monoclonal antibodies and other immunopotentiating agents, such as corynebacterium parvum (C. parvum) (Kopp, W. C., et al., 1994, Cancer Chemotherapy and Biol. Response Modifiers 15:226-286). There is little doubt that immunotherapy of cancer as it stands is falling short of the hopes invested in it. Although numerous immunotherapeutic approaches have been tested, few of these procedures have proved to be effective as the sole or even as an adjunct form of cancer prevention and treatment.
2.3.1. Interleukins (IL-2, IL-4 and IL-6)
IL-2 has significant antitumor activity in a small percentage of patients with renal cell carcinoma and melanoma. Investigators continue to search for IL-2 based regimens that will increase the response rates in IL-2 responsive tumors, but, for the most part, have neither defined new indications nor settled fundamental issues, such as whether dose intensity is important in IL-2 therapy (Kopp, W. C., et al., 1994, Cancer Chemotherapy and Biol. Response Modifiers 15:226-286). Numerous reports have documented IL-2 associated toxicity involving increased nitrate levels and the syndrome of vascular leak and hypotension, analogous to septic shock. In addition, an increased incidence of nonopportunistic bacterial infections and autoimmune complications are frequently accompanied by the antitumor response of IL-2 (Kopp, W. C., et al., 1994, Cancer Chemotherapy and Biol. Response Modifiers 15:226-286).
IL-4 and IL-6 are also being tested as antitumor agents either directly or through immunomodulating mechanisms. Dose-limiting toxicities have been observed with both agents in Phase I clinical trials (Gilleece, M. H., et al., 1992, Br. J. Cancer 66:204-210, Weber, J., et al., 1993, J. Clin. Oncol. 11:499-506).
2.3.2. Tumor Necrosis Factor
The toxicity of systemically administered TNF seriously limits its use for the treatment of cancer. TNF has been most effective when used for regional therapy, in which measures, such as limb isolation for perfusion, are taken to limit the systemic dose and hence the toxicity of TNF. Dose-limiting toxicity of TNF consist of thrombocytopenia, headache, confusion and hypotension (Mittleman, A., et al., 1992, Inv. New Drugs 10:183-190).
2.3.3. Interferons
The activity of IFN-α has been described as being modest in a number of malignancies, including renal cell carcinoma, melanoma, hairy cell leukemia low-grade non-Hodgkin's lymphoma, and others. Higher doses of IFN-α are usually associated with higher response rates in some malignancies, but also cause more toxicity. In addition, more and more reports indicate that relapses after successful interferon therapy coincide with formation of neutralizing antibodies against interferon (Ouesada, J. R., et al., 1987, J. Interferon Res. 67:678.
2.4. Pharmacokinetic Models for Anticancer Chemotherapeutic and Immunotherapeutic Drugs: Extrapolation and Scaling of Animal Data to Humans
The ethical and fiscal constraints which require the use of animal models for most toxicology research also impose the acceptance of certain fundamental assumptions in order to estimate dose potency in humans from dose-response data in animals. Interspecies dose-response equivalence is most frequently estimated as the product of a reference species dose and a single scaling ratio based on a physiological parameter such as body weight, body surface area, maximum life span potential, etc. Most frequently, exposure is expressed as milligrams of dose administered in proportion to body mass in kilograms (mg kg−1). Body mass is a surrogate for body volume, and therefore, the ratio milligrams per kilogram is actually concentrations in milligrams per liter (Hirshaut, Y., et al., 1969, Cancer Res. 29:1732-1740). The key assumptions which accompany this practice and contribute to its failure to accurately estimate equipotent exposure among various species are: i) that the biological systems involved are homogeneous, “well-stirred volumes” with specific gravity equal to 1.0; ii) that the administered compounds are instantly and homogeneously distributed throughout the total body mass; and iii) that the response of the biological systems is directly proportional only to the initial concentration of the test material in the system. As actual pharmacokinetic conditions depart from these assumptions, the utility of initial concentration scaling between species declines.
Through pharmacokinetics, one can study the time course of a drug and its metabolite levels in different fluids, tissues, and excreta of the body, and the mathematical relationships required to develop models to interpret such data. It, therefore, provides the basic information regarding drug distribution, availability, and the resulting toxicity in the tissues and hence, specifies the limitation in the drug dosage for different treatment schedules and different routes of drug administration. The ultimate goal of the pharmacokinetic studies of anticancer drugs is thus to offer a framework for the design of optimal therapeutic dosage regimens and treatment schedules for individual patients.
The currently utilized guidelines for prescription have evolved gradually without always having a complete and explicit justification. In 1966, Freireich and co-workers proposed the use of surface area proportions for interspecies extrapolation of the acute toxicity of anticancer drugs. This procedure has become the method of choice for many risk assessment applications (Freireich, E. J., et al., 1966, Cancer Chemotherapy Rep. 50:219-244). For example, surface area scaling is the basis of the National Cancer Institute's interspecies extrapolation procedure for anti-cancer drugs (Schein, P. S., et al., 1970, Clin. Pharmacol. Therap. 11:3-40; Goldsmith, M. A., et al., 1975, Cancer Res. 35:1354-1364). In accepting surface area extrapolation, the tenuous basis for initial concentration scaling has been replaced by an empirical approach. The basic formula used for estimating prescription of cancer chemotherapy per body surface area (BSA) is BSA=k×kg2/3, in which k is a constant that differs for each age group and species. For example, the k value for adult humans is 11, while for mice it is 9 (See Quiring, P., 1955, Surface area determination, in Glasser E. (ed.) Medical Physics I Chicago: Medical Year Book, p. 1490 and Vriesendorp, H. M., 1985, Hematol. (Supplm. 16) 13:57-63). The major attraction of expressing cancer chemotherapy per m2 BSA appears to be that it offers an easily remembered simplification, i.e., equal doses of drug per m2 BSA will produce approximately the same effect in comparing different species and age groups. However, simplicity is not proof and alternative methods for estimating prescription of anticancer drugs appear to have a better scientific foundation, with the added potential for a more effective use of anticancer agents (Hill, J. A., et al., 1989, Health Physics 57:395-401).
The effectiveness of an optimal dose of a drug used in chemotherapy and/or immunotherapy can be altered by various factors, including tumor growth kinetics, drug resistance of tumor cells, total-body tumor cell burden, toxic effects of chemotherapy and/or immunotherapy on cells and tissues other than the tumor, and distribution of chemotherapeutic agents and/or immunotherapeutic agents within the tissues of the patient. The greater the size of the primary tumor, the greater the probability that a large number of cells (drug resistant and drug sensitive) have metastasized before diagnosis and that the patient will relapse.
Some metastases arise in certain sites in the body where resistance to chemotherapy is based on the limited tissue distribution of chemotherapeutic drugs administered in standard doses. Such sites act as sanctuaries that shield the cancer cells from drugs that are circulating in the blood; for example, there are barriers in the brain and testes that impede drug diffusion from the capillaries into the tissue. Thus, these sites may require special forms of treatment such as immunotherapy, especially since immunosuppression is characteristic of several types of neoplastic diseases.